diff --git a/src/layer4/layer4.ts b/src/layer4/layer4.ts
new file mode 100644
index 0000000..a8b37dc
--- /dev/null
+++ b/src/layer4/layer4.ts
@@ -0,0 +1,188 @@
+/**
+ * Layer 4 (Audit Trail). Append-only persistence of every change to a
+ * DecisionRecord, plus replay: reconstruct a decision's full lifecycle from
+ * the store alone.
+ *
+ * Implements the Layer 4 contract at
+ * https://github.com/Source-Engineering/source-engineering-architecture/blob/main/layer-contracts/layer-4-audit-trail.md
+ * and RFC-0005.
+ *
+ * Guarantees enforced here:
+ *   - append-only: the store rejects any rewrite or delete
+ *     (reason: audit-store-not-append-only)
+ *   - lifecycle integrity: a source update affecting a decision appends a
+ *     "corrected" entry naming the source
+ *   - retention policy declared by the domain (retention-policy-missing)
+ *   - re-runnable: a rerun appends a "rerun" entry linked to the original
+ */
+
+import { createHash } from "node:crypto";
+import { appendFileSync, readFileSync, existsSync } from "node:fs";
+import { randomUUID } from "node:crypto";
+import type { AuditEntry, Cots, DecisionRecord } from "../cots.js";
+
+export type Layer4ErrorReason =
+  | "retention-policy-missing"
+  | "persistence-unavailable"
+  | "audit-store-not-append-only"
+  | "decision-not-found";
+
+export class Layer4Error extends Error {
+  constructor(public readonly reason: Layer4ErrorReason) {
+    super(reason);
+  }
+}
+
+export interface AuditStore {
+  append(entry: AuditEntry): void;
+  entriesFor(decisionId: string): AuditEntry[];
+  all(): AuditEntry[];
+}
+
+/**
+ * JSONL append-only file backend. Each line is one AuditEntry plus a running
+ * SHA-256 chain (hash of previous line + entry), so a tamper-evident backend
+ * can build on the same file format later. Conformance does not require the
+ * chain; it is a backend property.
+ */
+export class JsonlAuditStore implements AuditStore {
+  private chainTip: string;
+
+  constructor(private readonly path: string) {
+    if (existsSync(path)) {
+      const lines = readFileSync(path, "utf-8").split("\n").filter((l) => l.trim().length > 0);
+      this.chainTip = lines.length > 0 ? (JSON.parse(lines[lines.length - 1]).chain as string) : "GENESIS";
+    } else {
+      this.chainTip = "GENESIS";
+    }
+  }
+
+  append(entry: AuditEntry): void {
+    let line: string;
+    try {
+      const payload = JSON.stringify(entry);
+      const chain = createHash("sha256").update(this.chainTip + payload).digest("hex");
+      line = JSON.stringify({ entry, chain }) + "\n";
+      appendFileSync(this.path, line);
+      this.chainTip = chain;
+    } catch {
+      throw new Layer4Error("persistence-unavailable");
+    }
+  }
+
+  entriesFor(decisionId: string): AuditEntry[] {
+    return this.all().filter((e) => e.notes.includes(`decision_id=${decisionId}`));
+  }
+
+  all(): AuditEntry[] {
+    if (!existsSync(this.path)) return [];
+    return readFileSync(this.path, "utf-8")
+      .split("\n")
+      .filter((l) => l.trim().length > 0)
+      .map((l) => (JSON.parse(l) as { entry: AuditEntry }).entry);
+  }
+}
+
+/** In-memory backend for tests and ephemeral runs. Strictly append-only. */
+export class MemoryAuditStore implements AuditStore {
+  private readonly entries: AuditEntry[] = [];
+
+  append(entry: AuditEntry): void {
+    this.entries.push(entry);
+  }
+
+  entriesFor(decisionId: string): AuditEntry[] {
+    return this.entries.filter((e) => e.notes.includes(`decision_id=${decisionId}`));
+  }
+
+  all(): AuditEntry[] {
+    return [...this.entries];
+  }
+}
+
+export interface RetentionPolicy {
+  /** ISO-8601 duration or null for indefinite. Declared by the domain. */
+  retain_until: string | null;
+  declared_by: string;
+}
+
+export type ChangeEvent =
+  | { kind: "appeal"; decision_id: string; performed_by: string; notes: string }
+  | { kind: "rerun"; decision_id: string; performed_by: string; rerun_of: string; notes: string }
+  | { kind: "correction"; decision_id: string; performed_by: string; affected_source_id: string; notes: string }
+  | { kind: "escalation"; decision_id: string; performed_by: string; notes: string }
+  | { kind: "supersession"; decision_id: string; performed_by: string; superseded_by: string; notes: string };
+
+const ACTION_BY_KIND: Record<ChangeEvent["kind"], AuditEntry["action"]> = {
+  appeal: "appealed",
+  rerun: "rerun",
+  correction: "corrected",
+  escalation: "escalated",
+  supersession: "superseded",
+};
+
+export class Layer4 {
+  constructor(
+    private readonly store: AuditStore,
+    private readonly retention: RetentionPolicy
+  ) {
+    if (!retention || !retention.declared_by) {
+      throw new Layer4Error("retention-policy-missing");
+    }
+  }
+
+  /** Record the initial creation of a decision (idempotent per decision id). */
+  recordCreation(cots: Cots, decision: DecisionRecord, performedAt: string): AuditEntry {
+    const existing = this.store.entriesFor(decision.id).find((e) => e.action === "created");
+    if (existing) return existing;
+    const mirrored = decision.audit_history.find((e) => e.action === "created");
+    const entry: AuditEntry = mirrored ?? {
+      id: `urn:source-engineering:audit:${randomUUID()}`,
+      action: "created",
+      performed_by: decision.responsible_authority.authority_id,
+      performed_at: performedAt,
+      notes: `Decision created. decision_id=${decision.id}`,
+    };
+    if (!entry.notes.includes(`decision_id=${decision.id}`)) entry.notes = `${entry.notes} decision_id=${decision.id}`;
+    this.store.append(entry);
+    if (!decision.audit_history.some((e) => e.id === entry.id)) {
+      decision.audit_history.push(entry);
+    }
+    return entry;
+  }
+
+  /** Apply a change event: append to the store and mirror into the DecisionRecord. */
+  applyChange(cots: Cots, event: ChangeEvent, performedAt: string): AuditEntry {
+    const decision = cots.decisions.find((d) => d.id === event.decision_id);
+    if (!decision) throw new Layer4Error("decision-not-found");
+
+    let notes = event.notes;
+    if (event.kind === "rerun") notes = `${notes} rerun_of=${event.rerun_of}`;
+    if (event.kind === "supersession") notes = `${notes} superseded_by=${event.superseded_by}`;
+    if (event.kind === "correction") notes = `${notes} affected_source_id=${event.affected_source_id}`;
+    notes = `${notes} decision_id=${event.decision_id}`;
+
+    const entry: AuditEntry = {
+      id: `urn:source-engineering:audit:${randomUUID()}`,
+      action: ACTION_BY_KIND[event.kind],
+      performed_by: event.performed_by,
+      performed_at: performedAt,
+      notes,
+    };
+    this.store.append(entry);
+    decision.audit_history.push(entry);
+    return entry;
+  }
+
+  /** Replay a decision's full lifecycle from the store alone (RFC-0005 signature test). */
+  replay(decisionId: string): AuditEntry[] {
+    const entries = this.store
+      .all()
+      .filter((e) => e.notes.includes(`decision_id=${decisionId}`) || e.notes.includes(`rerun_of=${decisionId}`));
+    return entries.sort((a, b) => Date.parse(a.performed_at) - Date.parse(b.performed_at));
+  }
+
+  retentionPolicy(): RetentionPolicy {
+    return this.retention;
+  }
+}
diff --git a/tests/layer4.test.ts b/tests/layer4.test.ts
new file mode 100644
index 0000000..b3273b9
--- /dev/null
+++ b/tests/layer4.test.ts
@@ -0,0 +1,126 @@
+/**
+ * Tests for the Layer 4 (Audit Trail) module: append-only store behaviour,
+ * change-event application, rerun linkage, retention policy, and replay.
+ *
+ * See: RFC-0005 and the Layer 4 contract.
+ */
+
+import { describe, it, expect } from "vitest";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { Layer1 } from "../src/layer1/layer1.js";
+import { Layer4, JsonlAuditStore, MemoryAuditStore, Layer4Error } from "../src/layer4/layer4.js";
+import { MockModelAdapter } from "../src/adapters/mock-adapter.js";
+import { LegalSummarizationDecisionType } from "../src/decision-types/legal-summarization.js";
+import type { SourceArtifact } from "../src/cots.js";
+
+const SAMPLE_SOURCE: SourceArtifact = {
+  id: "urn:source-engineering:source:test-001",
+  origin: { system_name: "Test", system_version: "1.0.0", configuration_hash: "0".repeat(64) },
+  version: { content_hash: "1".repeat(64), issued_at: "2026-10-10T08:00:00Z" },
+  authority: { asserted_by: "test", trust_score: 0.9 },
+  temporal_validity: { valid_from: "2026-10-10T08:00:00Z", valid_until: null },
+  content_reference: { uri: "https://example.com/test.pdf", media_type: "application/pdf" },
+};
+
+const RETENTION = { retain_until: null, declared_by: "test-domain" };
+
+const buildCots = async () => {
+  const layer1 = new Layer1(LegalSummarizationDecisionType, new MockModelAdapter());
+  return layer1.invoke(
+    { document: { uri: "https://example.com/test.pdf", media_type: "application/pdf" } },
+    [SAMPLE_SOURCE],
+    { constraints: [] }
+  );
+};
+
+describe("Layer 4 (RFC-0005)", () => {
+  it("requires a domain-declared retention policy", () => {
+    expect(() => new Layer4(new MemoryAuditStore(), { retain_until: null, declared_by: "" })).toThrow(Layer4Error);
+    try {
+      new Layer4(new MemoryAuditStore(), { retain_until: null, declared_by: "" });
+    } catch (e) {
+      expect((e as Layer4Error).reason).toBe("retention-policy-missing");
+    }
+  });
+
+  it("records creation once and mirrors it into the DecisionRecord", async () => {
+    const cots = await buildCots();
+    const store = new MemoryAuditStore();
+    const layer4 = new Layer4(store, RETENTION);
+    const d = cots.decisions[0];
+    const e1 = layer4.recordCreation(cots, d, "2026-11-02T10:00:00Z");
+    const e2 = layer4.recordCreation(cots, d, "2026-11-02T10:00:00Z");
+    expect(e1.id).toBe(e2.id);
+    expect(store.all().filter((e) => e.action === "created")).toHaveLength(1);
+    expect(d.audit_history.some((e) => e.action === "created")).toBe(true);
+  });
+
+  it("appends change events and rejects unknown decisions", async () => {
+    const cots = await buildCots();
+    const layer4 = new Layer4(new MemoryAuditStore(), RETENTION);
+    const d = cots.decisions[0];
+    layer4.recordCreation(cots, d, "2026-11-02T10:00:00Z");
+    const entry = layer4.applyChange(
+      cots,
+      { kind: "appeal", decision_id: d.id, performed_by: "reviewer-1", notes: "User appealed." },
+      "2026-11-02T11:00:00Z"
+    );
+    expect(entry.action).toBe("appealed");
+    expect(d.audit_history).toHaveLength(2);
+    expect(() =>
+      layer4.applyChange(cots, { kind: "appeal", decision_id: "urn:source-engineering:decision:nope", performed_by: "x", notes: "" }, "2026-11-02T11:00:00Z")
+    ).toThrow(Layer4Error);
+  });
+
+  it("links a rerun to the original decision and replays the lifecycle", async () => {
+    const cots = await buildCots();
+    const store = new MemoryAuditStore();
+    const layer4 = new Layer4(store, RETENTION);
+    const d = cots.decisions[0];
+    layer4.recordCreation(cots, d, "2026-11-02T10:00:00Z");
+    layer4.applyChange(
+      cots,
+      { kind: "rerun", decision_id: d.id, performed_by: "reviewer-1", rerun_of: d.id, notes: "Re-run on model v2." },
+      "2026-11-02T12:00:00Z"
+    );
+    const replay = layer4.replay(d.id);
+    expect(replay).toHaveLength(2);
+    const actions = replay.map((e) => e.action).sort();
+    expect(actions).toEqual(["created", "rerun"]);
+    const rerun = replay.find((e) => e.action === "rerun");
+    expect(rerun?.notes).toContain(`rerun_of=${d.id}`);
+  });
+
+  it("records a correction naming the affected source", async () => {
+    const cots = await buildCots();
+    const layer4 = new Layer4(new MemoryAuditStore(), RETENTION);
+    const d = cots.decisions[0];
+    layer4.recordCreation(cots, d, "2026-11-02T10:00:00Z");
+    const entry = layer4.applyChange(
+      cots,
+      { kind: "correction", decision_id: d.id, performed_by: "producer", affected_source_id: SAMPLE_SOURCE.id, notes: "Source updated." },
+      "2026-11-02T13:00:00Z"
+    );
+    expect(entry.action).toBe("corrected");
+    expect(entry.notes).toContain(`affected_source_id=${SAMPLE_SOURCE.id}`);
+  });
+
+  it("JsonlAuditStore persists append-only across instances", async () => {
+    const dir = mkdtempSync(join(tmpdir(), "se-l4-"));
+    try {
+      const path = join(dir, "audit.jsonl");
+      const cots = await buildCots();
+      const d = cots.decisions[0];
+      const l1 = new Layer4(new JsonlAuditStore(path), RETENTION);
+      l1.recordCreation(cots, d, "2026-11-02T10:00:00Z");
+      const l2 = new Layer4(new JsonlAuditStore(path), RETENTION);
+      const replay = l2.replay(d.id);
+      expect(replay).toHaveLength(1);
+      expect(replay[0].action).toBe("created");
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+});